Field ionization is capable of ionizing large biological molecules without excessively fragmenting them. This is an important characteristic for the mass spectral analysis of biological samples containing a large number of different compounds. Field ion mass spectrometry, however, has not yet achieved its full potential for these applications, primarily because of its lack of sensitivity and the delicate nature of its operation. Recent advances in the design and fabrication of field ion sources, however, appear to be reducing these undesirable field ion characteristics. The purpose of this proposal is to rapidly develop and perfect the microfabrication techniques for producing advanced field ion source designs and to field test these new ion sources on mass spectrometers dedicated to medically oriented problems. The recently developed volcano field ion source has already demonstrated its potential capabilities of operating at very high pressures and with high efficiency. The microfabrications technology which produced this source can produce further important improvements. Specifically, we feel that the shape of the volcano orifice can be constructed so that the source can operate at atmospheric pressure, thus permitting the direct monitoring of breath and air environments. Also, we believe the source is capable of operating with liquid samples, thereby allowing body fluids to be analyzed without any sample pretreatment. To accomplish this will require developing new structural arrangements between the volcano and the counterelectrode. These and other related developments are necessary to apply the full potential of field ion mass spectrometry to medical diagnosis. The expertise of both the microfabrication engineers and the medical instrumentalists can be combined to produce important innovations for applied medicine.